Immunosuppressive effect induced by Actinobacillus actinomycetemcomitans: effect on immunoglobulin production and lymphokine synthesis

The soluble sonicated extract (SE) from Actinobacillus actinomycetemcomitans inhibited primary T cell-dependent antibody responses in vivo. The production of IgG and IgM to sheep red blood cells (SRBC) was depressed when mice were treated with high concentrations of SE plus SRBC. Preinjection of SE 3 days prior to SRBC completely inhibited IgG production. SE plus SRBC-primed mice showed markedly depressed CD4/CD8 ratios relative to phosphate-buffered saline plus SRBC- or SRBC-immunized mice. SE-sensitized mice showed low blastogenic activity to concanavalin A (Con A) depending on sensitized periods induced by SE. This inhibitory mechanism was, in part, clarified by a suppression of IL-2 synthesis, IL-2 receptor expression and IL-6 secretion by the splenic T cells stimulated with Con A. These results support the hypothesis that the severe infection of A. actinomycetemcomitans suppresses the immune response by affecting CD4/CD8 ratios, followed by lymphokine production and finally antibody responses.